Work of this character is common in the field and on construction sites in connection with surveying, construction work, and building of houses and ships. Similar tasks are common in connection with erection and adjusting of machines and measuring apparatus, so called optical tooling.
For such tasks it is now most common to employ instruments based on a sighting telescope and a movable object, for instance a levelling staff, or some other measuring object having a clearly visible pattern.
In operating on greater distances it is, however, for practical reasons necessary that two persons are involved, viz. one person at the sighting instrument, and one other person moving the measuring object around. If all operations, including the read-out, can be performed from the position of the measuring object one person would be able to perform both read-out and moving the measuring object, and the work would therefore become considerably more effective.
In high precision optical tooling the inevitable heat and vibration originating from the operator would disturb the precise aligning of the telescope instrument. It would therefore be an advantage to have the operator physically separated from the instrument, the position of which defines the desired plane or line.
The employment of light beams from gas lasers emitted in a given direction or in a given plane is also known, but in view of the dangers pertaining to laser light and the practical difficulties encountered, inter alia in the current supply to lasers, it would be advantageous to be able to obviate these components.
Instruments operating by means of image pairs having oppositely oriented components are known from the British Pat. Nos. 684,292 and 1,235,664. Their function relies on oppositely oriented images of a measuring object being displaced in each its direction by a unilateral displacement of the measuring object from a given plane or line. This relative displacement of the images is observed by an eye, possibly enlarged by means of a telescope or the like.
In instruments according to the first mentioned patent an image pair is formed by semi-transparent mirrors located in both focal planes of a lens or in a focal plane of a concave mirror. The formation of the image pair is in other words dependent on a depiction by a focussing system which means that the images in the case of large measuring distances must be highly reduced relative to the measuring object in order to obtain reasonable dimensions of the optical system. This is very disadvantageous with a view to the faithfulness of the instrument.
British Pat. No. 1,235,664 describes an instrument which by means of reflections in plane surfaces forms a vertically inverted image in natural size of the measuring object. On regarding this image together with the direct sight through the instrument the same effect is obtained as by an image pair having oppositely oriented components and an exactitude corresponding to work with a sighting telescope. An instrument according to that specification is, however, to be employed in the same manner and with principally the same lay-out as the sighting telescope, since the measuring object is to be viewed through the instrument so that the measuring operation cannot be performed by a person on the position of the measuring object.